The present invention generally relates to vibration isolating apparatuses, and more particularly to a vibration isolating apparatus which reduces effects of an earthquake vibration on an object which is placed on the vibration isolating apparatus so as to effectively prevent damage to the object by the earthquake.
Generally, an art object, a precision machine such as a computer, a tank for medical fluid, a shelf for keeping chemicals and the like must be prevented to the utmost from coming down, cracking, overflowing, falling and the like when an earthquake occurs. Accordingly, a vibration isolating apparatus has been conventionally proposed to prevent such an object of value, a precision machine, a dangerous object and the like from being damaged by the earthquake.
The conventional vibration isolating apparatus generally comprises a lower bed which is fixed to a floor of a building, an upper bed which is slideably provided on the lower bed and has an object placed thereon, and four coil springs which pull the outer periphery of the upper bed in four mutually perpendicular directions on a horizontal plane. When the earthquake occurs, a roll on the floor is resiliently absorbed directly by the coil springs, and the roll is prevented from being transmitted to the upper bed.
However, according to the conventional vibration isolating apparatus having the construction described above, the four coil springs apply mutually equal pulling forces on the outer periphery of the upper bed so that a center position of the upper bed coincides with a center position of the lower bed. The coil springs are maintained in a stretched state where the coil springs pull on the outer periphery of the upper bed, and the coil springs having a relatively large spring constant must be mounted in the stretched state. For this reason, there are disadvantages in that it is difficult to mount the coil springs and the mounting efficiency is poor.
When it is assumed that the earthquake vibration has an input period T1 and the vibration of the upper bed has a natural period T2, the input period T1 must be smaller than the natural period T2, and the vibration isolation effect is large when the natural period T2 is large, where the natural period T2 is determined by the spring constant of the coil spring and the mass of the object which is placed on the upper bed. But in order to improve the vibration isolation effect of the coil springs, it is necessary to make the spring constant small, increase the tension in the coil springs and increase the displaceable range of the lower bed with respect to the upper bed which should remain stationary even when there is a roll on the floor. Hence, according to the conventional vibration isolation apparatus which absorbs the vibration by the coil springs, a large mounting space is required when the vibration isolation effect is to be improved, but on the other hand, it is impossible to obtain a sufficiently large tension in the coil springs within a limited mounting space. As a result, there is a limit to improving the vibration isolation effect.
In the case where the object placed on the upper bed is a computer or the like which has a large weight, it is difficult to manually lift the heavy object and place it on the upper bed. But inside the building, it is difficult to use a lifting apparatus such as a crane. Hence, there is a disadvantage in that it is difficult to lift and place a heavy object on the upper bed of the vibration isolation apparatus. Furthermore, in the case where cables are provided under the floor and an electric machine is to be placed on the upper bed, it is necessary to provide on the upper and lower beds cutout parts for passing the cables so as to connect the cables to terminals which are located on a lower surface of the electric machine, for example. In addition, when the object to be placed on the upper bed is bulky, the overall size of the vibration isolation apparatus becomes large and heavy, and there is a disadvantage in that it is difficult to carry and move the vibration isolation apparatus.